We have recently discovered that hemoglobin is enriched in myoendothelial junctions, the anatomical location where endothelial cells and smooth muscle cells make contact in the resistance arteries. This was a significant finding because it demonstrated that hemoglobin had an important and active role outside of erythrocytes. This protein is one of only a few truly polarized proteins to be localized to endothelial-derived myoendothelial junctions, and the siRNA-induced decrease in the amount of the protein significantly altered arterial reactivity, including constriction to phenylephrine and dilation to acetylcholine. The mechanism we derived was based on evidence indicating that monomeric hemoglobin is a potent scavenger of nitric oxide, and that endothelial nitric oxide synthase (eNOS) and hemoglobin were found to be in a macromolecular complex. Based on this work, as well as a plethora of strong preliminary data, we hypothesize that hemoglobin at the myoendothelial junction is a novel regulator of nitric oxide signaling which can impact blood pressure regulation. We will test this hypothesis using two specific aims: 1.) investigate the effects of endothelial hemoglobin a gene ablation/over-expression on arterial function and 2.) elucidate how AHSP and eNOS regulate hemoglobin expression and dioxygenase activity at the MEJ. These aims will be elucidated using studies focused first on a floxed hemoglobin mouse as well as a hemoglobin a over-expressing mouse to determine the effects of deletion/over-expression of this protein in endothelium on arterial reactivity, whole tissue blood flow, peripheral resistance and blood pressure. In addition, a human model of the disease alpha thalassemia where 2 alleles of hemoglobin a are deleted will be used to study the effects of this genome-wide heterozygous deletion on the vasculature. Next we will investigate how the hemoglobin chaperone hemoglobin stabilizing protein (AHSP) may traffic hemoglobin to the myoendothelial junction or act directly as a regulator of the hemoglobin a redox state, altering the ability of nitric oxide to bind. The sum of this proposal unites and builds on the dat obtained from our previous R01 by allowing us for the first time to ask the direct question as to the function of the myoendothelial junction in intact animals. Indeed, we believe part of the answers could provide the basis for a completely new understanding of blood pressure control by the peripheral vasculature, as well as the derivation of unexplored pharmacological targets for control of hypertension.